1. Field of Invention
The present invention relates to fuel cells, and more particularly to a monitoring system for fuel cell stack by monitoring the output voltage of each cell unit of the fuel cell stack to ensure the fuel cell stack working under an optimum condition.
2. Description of Related Arts
Electrochemical fuel cell is a kind of electrochemical energy conversion device which is capable of converting the hydrogen and oxidant into electrical energy. The core component of such device is the membrane electrode assembly (MEA). The MEA comprises a proton exchange membrane sandwiched by two porous sheets made of conductive material such as carbon tissue. Catalyst like metal platinum powder, adapted for facilitating the electrochemical reaction, are evenly and granularly provided on two carbon tissue sheets to form two catalytic interfaces. Furthermore, electrically conductible members are provided on two sides of the MEA to form a cathode and an anode in such a manner that electrons generated due to the electrochemical reaction are capable of being lead out through an electrical circuit.
The anode of the MEA is supplied with fuel, such as hydrogen, for initiating the electrochemical reaction. The fuel is forced through the porous and diffused carbon tissue sheet, and loses electrons to form positive ions on the catalytic interface due to the electrochemical reaction. The positive ions are capable of transferably penetrating the proton exchange membrane to reach the cathode. On the other hand, an oxidant-containing gas, such as oxygen and air, is supplied to the cathode of the MEA, wherein the oxidant-containing gas penetrates the porous and diffused carbon tissue sheet to generate negative ions due to the electrochemical reaction occurred on the catalytic interface. Finally, the positive ions transferred from the anode meets the negative ions to form reaction product.
The fuel cells employ hydrogen as the fuel and oxygen containing air (or pure oxygen) as the oxidant. The fuel hydrogen forms hydrogen positive ions (protons) at the anode portion due to the electrochemical reaction. The proton exchange membrane is capable of facilitating the hydrogen positive ions to migrate from the anode to the cathode. In addition, the proton exchange member also separates the hydrogen gas flow and the oxygen gas flow to prevent the mixture of hydrogen and oxygen as well as the explosive reaction therefor.
Oxygen obtains electrons from the catalytic interface to form negative ions at the cathode portion of the fuel cell due to the electrochemical reaction. The negative ions reacts with the hydrogen positive ions transferred from the anode portion to form reaction water product. In the fuel cells which utilize the hydrogen as the fuel and oxygen containing air as oxidant, the electrochemical reaction can be expressed by the following formula:Anode: H2→2H++2eCathode: ½O2+2H++2e→H2O
In the typical proton exchanging membrane fuel cell system, the MEA is disposed between two electrically conductible electrode plates wherein the contacting interface of each electrode plate defines at least one flowing channel. The flowing channel could be embodied by conventional mechanical method such as pressure casting, punching, and mechanical milling. The electrode plate could be embodied as metal electrode plate or graphite electrode plate. So the flowing channels provided on the electrode plate are capable of respectively directing fuel and oxidant into the anode portion and the cathode portion positioned on opposite sides of the MEA. For a single fuel cell structure, only one MEA is provided and disposed between an anode plate and a cathode plate. Here, the anode plate and the cathode plate not only are embodied as current-collecting device, but also as a supporting device for securely holding the MEA. The flowing channels provided on the electrode plates are capable of delivering fuel and oxidant to the catalytic interfaces of the anode and cathode, and removing the water discharged due to the electrochemical reaction of fuel cell.
To increase the overall power output of the proton exchanging membrane fuel cell, two or more fuel cells are electrically connected in series in a stacked manner or a successive manner to form a fuel cell stack. In such stacked series manner, each electrode plate provides flowing channels on opposite side of plate respectively wherein one side of the electrode plate is applied as an anode plate contacting with the anode interface of a MEA, while another side of the electrode plate is applied as a cathode plate contacting with the cathode interface of an adjacent MEA. That is to say, one side of such electrode plate serve as an anode plate for one fuel cell unit and the other side of plate serve as a cathode plate for the adjacent cell. This kind of structure is called bipolar plate. Conclusively, the fuel cell stack comprises a plurality of fuel cell units electrically connected with each other, and a pair of end plates disposed at two ends of such stack for securing the plurality of fuel cell units in position.
It is well known that fuel cell stack is used as power system for propelling vehicles and vessels, and for operating other electrically operated machines such as portable generators.
To support such powerful operator, a plurality of individual fuel cell units, commonly hundreds or thousands of fuel cell unit, is interconnected in series manner. As a result, a monitoring system is very important for monitoring the voltage output of each single fuel cell unit in order to prevent abnormal operation, such as overcurrent or excess working temperature of the fuel cell stack.
Accordingly, the overall output voltage of the fuel cell stack is determined by the accumulation of the outputs of the individual fuel cells electrically connected in a series manner. Therefore, when one of the individual fuel cells fails to operate, the overall performance of the fuel cell stack would be downgraded. In other words, it is crucial to monitor the performance of individual cell to ensure the overall performance of the fuel cell stack in good shape. Especially, when an electrode is disruptive, the voltage output of such electrode will reach an abnormal value, such as a value close to zero, or even a negative value. In contrary, the voltage output value of a normal fuel cell unit should be within a range between 0.5–1.2V. And more importantly, the extended service of such abnormal fuel cell unit would cause severe consequences. Therefore, it is necessary to monitor every single fuel cell unit of a fuel cell stack that, whenever certain fuel cell units monitored indicated an abnormal voltage value, the controlling system of such fuel cell stack would be able to provide alert or alarm signals and to shut down the whole system.
Shanghai Shenli Co. introduced an innovative device and method for monitoring voltage of individual fuel cell unit of a fuel cell stack, Chinese Patent No. 02136838.4. According to Shenli's invention, the live voltage output of each fuel cell unit could be directly measured and that once an abnormal voltage is detected, an alerter is provided for initiating an order for protecting the fuel cell stack.
Referring to the FIG. 1, the conventional monitoring device comprises a MCU (micro chip unit) processor which has an A/D converter and a plurality of switches, and a plurality of measuring lines extended from individual fuel cell unit of the fuel cell stack to electrically couple with the plurality of switches respectively, wherein each of the measuring lines is electrically connected to the A/D converter via a decoding matrix, such that a voltage signal is sent out from each of the measuring lines to the MCU processor through the decoding matrix for monitoring the voltage output of each of the individual fuel cells.
Theoretically, the above mentioned monitoring system would be effective in practice. However, in case the fuel cell stack to be monitored contains too many fuel cell units or there are several fuel cell stacks needed to be monitored simultaneously, such monitoring device would suffer some unavoidable drawbacks. First of all, the RS485 communicating mode is vulnerable in reliability and inefficient in transmitting speed. Second, as shown in FIG. 1, a plurality of corresponding decoding matrix circuit would be needed for facilitating the operation. Unfortunately, such decoding circuits are rather bulky and not suitable to be used for large scale fuel cell stacks. Third, since there is considerable number of measure lines, the leading wires for such monitoring system would be lengthy. Fourth, the prolonged leading wires are susceptible to be interfered by surrounding environments thus worsening the stability and reliability of the fuel cell stack. Finally, the assembly process of the monitoring system is rather complicated and awesome for common operators.